The Rankine cycle is a thermodynamic cycle in which heat is converted into work. The heat is supplied externally to a closed loop, typically with water as the working fluid. The Rankine cycle generates about 80% of all electric power used throughout the world, and is used by solar thermal, biomass, coal and nuclear power plants. Rankine-cycle power systems typically transform thermal energy into electrical energy. A conventional Rankine cycle power system employs the following four basic steps: (1) thermal energy is used, in a boiler, to turn water into steam; (2) the steam is sent through a turbine, which, in turn, drives an electric generator; (3) the steam is condensed back into water by discharging the remaining thermal energy in the steam to the environment; and (4) the condensate is pumped back to the boiler. In an ideal Rankine cycle, the expansion is isentropic (i.e., at constant entropy) and the evaporation and condensation processes are isobaric (i.e., at constant pressure). However, irreversibilities in real processes lower cycle efficiency. Those irreversibilities are primarily attributable to two factors: conversion of some energy into heat during expansion of the working fluid during step (2) of the cycle; and inefficiency caused by pressure drops in the heat exchangers during steps (1) and (3).
The efficiency of a Rankine cycle is a function of the physical properties of the working fluid. Without the pressure reaching supercritical levels for the working fluid, the temperature range over which the cycle can operate is quite small. For example, conventional turbine entry temperature limits are around 565° C. (the creep limit of stainless steel) and condenser temperatures are around 30° C. This gives a theoretical Carnot efficiency of about 63% compared with an actual efficiency of 42% for a modern coal-fired power station. Low turbine entry temperature (compared with an internal-combustion gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined-cycle gas turbine power stations. The working fluid in a Rankine cycle follows a closed loop and is re-used continually. While many working fluids can and have been used in the Rankine cycle, water is usually the fluid of choice because it is abundant, inexpensive, nontoxic, generally non-reactive, and possesses favorable thermodynamic properties. Organic Rankine cycles (ORCs) have been developed to enable recovery of energy from lower temperature sources, such as industrial waste heat, geothermal heat, solar ponds, etc. Working fluids in ORCs are organic, high molecular mass fluids having a liquid-vapor phase change (i.e., boiling point) at a lower temperature than the water-steam phase change at a given pressure. Using an ORC, heat from lower temperature sources can be converted to useful work that can be harnessed to generate electricity.
ORC technology can be used to recover energy from waste heat. For example, the technology can be applied to heat and power plants, industrial and farming processes (e.g., organic products fermentation), hot exhausts from ovens or furnaces, flue gas condensation, exhaust gases from vehicles, intercooling of a compressor, and the condenser of a power cycle. ORC technology can also be used to extract useful energy from biomass, geothermal heat sources, solar fields, etc.
ORCs are described in International Patent Publication No. WO 2014/124061, titled “Improved Organic Rankine Cycle Decompression Heat Engine,” published Aug. 14, 2014, the entire disclosure of which is hereby incorporated herein by this reference.